1. Field of the Invention
The present invention relates generally to light emitting diodes (LEDs) and more particularly to packages for high-power LEDs.
2. Description of the Prior Art
A light emitting diode (LED) is a semiconductor device that produces light when an electric current is passed therethrough. LEDs have many advantages over other lighting sources including compactness, very low weight, inexpensive and simple manufacturing, freedom from burn-out problems, high vibration resistance, and an ability to endure frequent repetitive operations. In addition to having widespread applications for electronic products as indicator lights and so forth, LEDs also have become an important alternative light source for various applications where incandescent and fluorescent lamps have traditionally predominated.
One improvement that has broadened the applicability of LEDs has been the use of phosphors in conjunction with LEDs. A phosphor is a luminescent material that, when excited by a light of a certain wavelength, produces a light at a different wavelength, thus modifying the output light of the LED. Thus, where a particular color is desired and that color cannot be produced by available LEDs (or cost effective LEDs), phosphors can be used as light “converters” to alter the color of the light produced by an available LED to the desired color.
For example, phosphors are now used with monochromatic LEDs to produce white light. Using phosphors to convert the light produced by an LED to white light has proven to be an excellent alternative to other conventional white light sources, including incandescent light sources and the direct red-green-blue (RGB) LED methods in which multiple monochromatic LEDs are combined in a RGB scheme to produce white light.
In a typical LED-based white light producing device, a monochromatic LED is encapsulated by a transparent material containing appropriate phosphors. In some systems, an LED that produces a monochromatic visible light is encapsulated by a material containing a compensatory phosphor. The wavelength(s) of the light emitted from the compensatory phosphor is compensatory to the wavelength of the light emitted by the LED such that the wavelengths from the LED and the compensatory phosphor mix together to produce white light. For instance, a blue LED-based white light source produces white light by using a blue light LED and a phosphor that emits a yellowish light when excited by the blue light emitted from the LED. In these devices the amount of the phosphor in the transparent material is carefully controlled such that only a fraction of the blue light is absorbed by the phosphor while the remainder passes unabsorbed. The yellowish light and the unabsorbed blue light mix to produce white light.
Another exemplary scheme uses an LED that produces light outside of the visible spectrum, such as ultraviolet (UV) light, together with a mixture of phosphors capable of producing either red, green, or blue light when excited. In this scheme, the light emitted by the LED only serves to excite the phosphors and does not contribute to the final color balance.
FIGS. 1 and 2 illustrate two alternative encapsulated light emitting devices that incorporate phosphors according to the prior art. FIG. 1 shows a schematic diagram of a light emitting device 10 having an LED 12 mounted on a substrate 14. The LED 12 is encapsulated by a phosphor-containing layer 16. A generally hemispherical and transparent spacer layer 15 is disposed between the LED 12 and the phosphor-containing layer 16. The spacer layer 15 is formed by dropping a viscous, transparent UV light-cured resin over the LED 12. The spacer layer 15 functions as a filler or spacer to improve the uniformity of the thickness of the phosphor-containing layer 16.
FIG. 2 shows a schematic diagram of another light emitting device 20 having an LED 22 mounted on a substrate 24. The LED 22 is encapsulated by a conformal phosphor-containing layer 26. As opposed to the light emitting device 10, the light emitting device 20 does not use a filler or spacer between the phosphor-containing layer 26 and the LED 22. The uniformity of the thickness of the phosphor-containing layer 26 is achieved instead by using a stenciling technique. Specifically, a patterned stencil is placed over the LED 22, and a curable silicone liquid matrix containing a phosphor material is stenciled on to the LED 22. When cured, the silicone liquid matrix containing the phosphor material forms the phosphor-containing layer 26 on the LED 22.
One problem with LED-based white light sources described above is the poor thermal stability of the phosphors. Specifically, exposure to high temperatures for extended periods tends to alter the chemical and physical properties of such phosphors causing performance deterioration. For instance, the light conversion efficiency can decline and the wavelength of output light can shift, both altering the balance of the light mixture and potentially diminishing the intensity of the overall output. For example, currently available phosphors are often based on oxide or sulfide host lattices including certain rare earth ions. Under prolonged high temperature conditions, these lattices decompose and change their optical behavior.
Other problems commonly found with LED-based white light sources are transient color changes and uneven color distributions, both caused by temperature gradients in the phosphor-containing material. Such behaviors often create an unsatisfactory illumination.
In addition to the issues presented above, a need exists for better light extraction from LED-based white light sources. Specifically, internal reflection at the LED/encapsulant interface prevents a fraction of the light generated inside the LED from escaping and can significantly reduce the light efficiency of the device (Lumens per Watt). The fraction of light prevented from escaping from the LED increases as a function of the difference between the indices of refraction of the materials on either side of the interface. Most semiconductor materials used for fabricating LEDs have refractive indices in the range of 2.5 to 3.5. On the other hand, the refractive indices of commonly used encapsulation materials, such as polymers and epoxies, are in the range of 1.5 to 1.55. In order to increase the light extraction efficiency, a material with a higher refractive index is needed to reduce reflective losses at the interface between the LED and the encapsulant.
Furthermore, high reliability is another requirement for encapsulation materials. Ideally speaking, an encapsulation material should have a coefficient of thermal expansion (CTE) close to that of the LED and the package materials. When this condition is difficult to meet, the elastic modulus of the encapsulation material should be low enough to reduce stresses on wire bonds and the LED.
Recent advances in semiconductor technology have made it possible to manufacture high-power LEDs that produce light with an output power of about 1 watt and above. With such high power outputs, devices incorporating such LEDs can run very hot. The above-mentioned thermal problems worsen with increasing temperature and therefore are particularly severe for devices that incorporate high-power LEDs with phosphors. Presently, the above-mentioned thermal problems have severely limited the allowable LED temperature to only about 125° C. and have limited the allowable thermal resistance of prior art lighting systems.
Given the importance of LEDs as light sources, particularly high-power LEDs, there is a need for improved LED packaging methods and materials to alleviate the above-identified problems by providing better heat resistance, higher light extraction efficiencies and higher reliabilities. Such packaging methods and materials will allow for devices with smaller packages and higher luminosities, which are critical for light source applications.